Research On Flexible Wearable Biosensors Based On Microneedle Structure

In the medical field,various bioelectric signals are used for biomedical research and diagnosis,d,it is necessary to record ECG accurately.Although the most commonly used wet electrodes in clinical can provide excellent signals,it is still challenged due to the following reasons.First,the conductive gel dehydrated over time will lead to the weakening of the obtained signal and the increase of noise,which is not conducive to long-term signal monitoring.Secondly,some patients will produce skin irritation and allergic reaction to conductive gel.Finally,the wet electrode needs special skin treatment before use,such as skin abrasion,hair cutting,which is cumbersome and irritating.To solve the above problems,the main work of the thesis is as follows:(1)A back exposure technique was used to fabricate the microneedle electrode,and the effects of different exposure times on the height and shape of the microneedle were studied.The microneedles prepared by backside exposure have high precision,good conformal adhesion,high consistency and can be mass-produced.The use of the double-layer parylene to wrap the SU-8microneedle structures solved the problem of poor adhesion between the metal and microneedles,as well as between the microneedles and substrate,achieving the fabrication of a highly-conformal,ultra-flexible microneedle array.The contact impedance between the electrode and the skin is also lower than that of the wet electrode.The signal testing results show that this microneedle electrode can stably record ECG signals.However,the height and shape of the microneedle are seriously affected by the thickness and exposure dose of photoresist spin coating,the height of the fabricated microneedle is relatively low,about 200 μm.Thus,the micromold method was explored,aiming at fabricating higher microneedles.(2)Thus,the micromold method was used to fabricate the microneedle structure,the height of the microneedle is about 500μm.The shape of the microneedle is only affected by the shape of the micromold.It is very convenient to adjust the micromold to adjust the shape of the microneedle.The morphology and impedance of the microneedles fabricated by the two methods were compared.The results showed that the impedance of the microneedles fabricated by the micromold method was lower,and the height of the microneedles was higher,with a sharper top.Therefore,silver nanoparticles(Ag),PEDOT: PSS(PPS),and Ag-PPS(Ag with PPS double layer material)were used to modify the microneedle prepared by the micromold method.The results show that the impedance of the electrode modified with double layer materials decreases most significantly,and the signal quality is also better than that of the wet electrode,with a higher signal-to-noise ratio.The advantages mainly come from two aspects: Primary,the PI microneedles physically pierce the stratum corneum,reducing skin impedance.Secondary,the electrochemically modified PPS comes into contact with the electrolyte,producing a Faraday reaction,which lowers charge transfer impedance.The mechanical test results showed that the modified microneedles could effectively penetrate the skin under 4 N of penetration force without bending or peeling and there was no significant skin irritation and allergic reactions after wearing them for about 6 hours.(3)To reduce electrode impedance and improve the quality of biological signal recording.This work studied the impact of various materials on electrode electrochemical performance.A flexible electrode was prepared,and five material modifications were performed using electrochemical deposition techniques to investigate the performance of different materials in reducing impedance and enhancing charge storage capacity.It was concluded that the electrochemical performance of the double-layer modified material was more excellent.In order to further explore the differences between materials,scanning electron microscope(SEM)was taken for electrodes modified with different materials.From the microscopic scale analysis,it was found that the single-layer electroplating Pt-black significantly increased the surface area of the electrode,which reduced electrode impedance while improving the mechanical stability of the material.In this paper,the polypeptide with good biocompatibility was deposited on the electrode by physical vapor deposition(PVD)method to enhance the biocompatible and its effect on the electrochemical performance of the electrode were further studied.